EMU impulse antenna

ABSTRACT

An electromagnetic energy source for emitting pulses of electromagnetic energy includes a sonde assembly having a first section axially aligned with, and spaced from, a second section. An energy storage capacitor of the sonde assembly includes an electrode mounted in each of the first section and the second section of the sonde assembly and operable to generate an electric field, and a capacitive charge storage medium mounted in each of the first section and the second section of the sonde assembly and surrounding each electrode. The sonde assembly further includes a fast-closing switch located between the electrodes of the first and second sections of the sonde assembly.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to imaging sub-surface structures,particularly hydrocarbon reservoirs and fluids therein, and moreparticularly to electromagnetic energy sources for electromagneticsurveying of sub-surface structures.

2. Description of the Related Art

Some electromagnetic (EM) surveying systems used in geophysics provideelectromagnetic energy for traveling through a subsurface hydrocarbonreservoir for electromagnetic imaging of the subsurface hydrocarbonreservoir. Multiple sources and receivers can be positioned either in abore that extends to the subsurface hydrocarbon reservoir or an earthsurface above the subsurface hydrocarbon reservoir. In this way, thedirection, velocity and saturation of injected fluids (e.g. during waterflood) can be monitored. The system can also be used to locate by-passedoil and detect high conductivity zones (e.g. fracture corridors andsuper-k zones) to provide early warning of water break-through. Suchoperations can assist in optimizing reservoir management, preventing oilbypass and thereby improving volumetric sweep efficiency and productionrates.

Some current EM systems in geophysics include an overly large antenna inorder to be able to generate a moderately low frequency signal out of asmall antenna. The apparent ‘aperture’ of the antenna (wavelength toantenna size ratio) can be problematic. Some current EM systems cannoteasily match impedance of the system to the geological matrix andincrease transmission efficiency. Some current EM systems usehigh-current cable to provide power to the EM transmitter. However,these systems have been shown to have difficulty transferring a crisphigh-current pulse from the power supply, down a low-loss cable, andthen matching that into the antenna and in addition, the high-currentcabling can also transmit a signal, which made the resultingmeasurements unclear.

SUMMARY OF THE DISCLOSURE

Embodiments of this disclosure combines a slow-wave antenna with energystorage and pulse forming elements to realize a high power, smallaperture transmitting antenna that is ideally suited for downholeelectromagnetic interrogation technologies, such as for electromagneticimaging of a subsurface hydrocarbon reservoir. Systems and methodsdescribed herein provide a transmitter that is compact, very high ininstantaneous power output and generates a very clean signal.

Embodiments of this disclosure provide a dipole antenna that bothincreases radiation resistance and transmitter efficiency. The length ofthe antenna is shorter than some current antennas because the dipoleantenna components are loaded with materials with relatively highdielectric permittivity (e), magnetic permeability (mu), or acombination of dielectric permittivity and magnetic permeability. Thematerial can be selected so that e and mu of such material optimizestransmitter impedance to match the external medium. This increases thecapacitance and inductance of the system and decreases the groupvelocity of the signal traveling along the antenna element, and canmaterially decrease the length of the antenna structure for a givenwavelength emitted.

In addition, the antenna elements of embodiments of this disclosure areused as capacitive energy storage elements, with each half of the dipoleinitially held at a high voltage relative to one another. A fast-closingswitch, such as a triggered spark gap, is provided between a pair ofsuch antennas to initiate pulsed transmission. The pair of antennas isbiased apart by a large voltage so that the structure can discharge in asingle massive current pulse and emit a very high power transient radiofrequency signal. Systems and methods of this disclosure thereforecombine energy storage, pulse formation and radiating elements into asingle structure, eliminating the need for impedance matching betweenseparate distributed components for these respective functions.

Systems and methods of this disclosure eliminate the problem of loadmatching between a power supply, cable or transmission-line and antenna.With the energy storage element and switch inside the transmittingantenna element, the cable between the two is eliminated, minimizingreflections and losses in the system.

In an embodiment of this disclosure, an electromagnetic energy sourcefor emitting pulses of electromagnetic energy includes a sonde assemblyhaving a first section axially aligned with, and spaced from, a secondsection. An energy storage capacitor of the sonde assembly includes anelectrode mounted in each of the first section and the second section ofthe sonde assembly and operable to generate an electric field, and acapacitive charge storage medium mounted in each of the first sectionand the second section of the sonde assembly and surrounding eachelectrode. A fast-closing switch is located between the electrodes ofthe first and second sections of the sonde assembly.

In alternate embodiments, the electromagnetic energy source can furtherinclude a high voltage power supply connected between the electrodes.Current limiting resistors can be located between the high voltage powersupply and the electrodes. The capacitive charge storage medium can beselected to provide a decrease of a group velocity of pulses ofelectromagnetic energy. The capacitive charge storage medium can beformed of a material that includes iron particles and an epoxy matrix.

In other alternate embodiments, the electromagnetic energy source canfurther include a plurality of electromagnetic energy sources emittingpulses of electromagnetic energy to travel through a subsurfacehydrocarbon reservoir. The electromagnetic energy source can be movableto a succession of locations in a well borehole for emitting the pulsesof electromagnetic energy at the locations for travel through asubsurface hydrocarbon reservoir. The sonde assembly can have aconductor member serving as a first conductor and the electrode canserve as a second conductor. The capacitive charge storage medium can belocated between the conductor member and the electrode. The conductormember can be electrically isolated from the electrode with thecapacitive charge storage medium.

In an alternate embodiment of this disclosure, a source for emitting thepulses of electromagnetic energy to travel through a subsurfacehydrocarbon reservoir for electromagnetic imaging of the subsurfacehydrocarbon reservoir includes a sonde assembly and fast closing switchattached to a wireline for travel in a well borehole to a depth ofinterest.

In another alternate embodiment of this disclosure, a system for usingpulses of electromagnetic energy to travel through a subsurfacehydrocarbon reservoir for electromagnetic imaging of the subsurfacehydrocarbon reservoir includes at least one electromagnetic energysource. Each electromagnetic energy source has a sonde assembly attachedto a wireline for travel in a well borehole to a depth of interest, thesonde assembly including a first section axially aligned with, andspaced from, a second section. An energy storage capacitor is formed bya conductor member extending along the sonde assembly, an electrode ismounted in each of the first section and the second section of the sondeassembly and serving as a second conductor, and a capacitive chargestorage medium is mounted in each of the first section and the secondsection of the sonde assembly between the sonde assembly and theelectrode. A fast-closing switch is located between one of the conductormembers and the electrodes of the first and second sections. A pluralityof electromagnetic sensors form a measure of a resulting signal from theelectromagnetic energy source.

In alternate embodiments, the plurality of electromagnetic sensors canbe mounted in a well tool lowered in sensor bore in the subsurfacehydrocarbon reservoir. The plurality of electromagnetic sensors can belocated in an array over an earth surface above the subsurfacehydrocarbon reservoir. The system can have a system control unit forstoring information relating to the resulting signal received by theplurality of electromagnetic sensors and for performing a computerizedanalysis of the resulting signal.

In yet another alternate embodiment of this disclosure, a method foremitting pulses of electromagnetic energy with an electromagnetic energysource includes providing an electromagnetic energy source having: asonde assembly including a first section axially aligned with, andspaced from, a second section; an energy storage capacitor including anelectrode mounted in each of the first section and the second section ofthe sonde assembly and a capacitive charge storage medium mounted ineach of the first section and the second section of the sonde assemblyand surrounding the electrode; and a fast-closing switch located betweenthe electrodes of the first and second sections. The method furtherincludes charging the energy storage capacitor to cause the fast-closingswitch to close and pulses of electromagnetic energy to be emitted fromthe electromagnetic energy source.

In alternate embodiments, the electromagnetic energy source can furtherinclude a high voltage power supply connected to the electrode of thefirst section and the electrode of the second section of the sondeassembly. The electromagnetic energy source can further include currentlimiting resistors located between the high voltage power supply andboth of the electrode of the first section and the electrode of thesecond section. The method can further include lowering theelectromagnetic energy source on a wireline in a well borehole to adepth of interest in a subsurface hydrocarbon reservoir.

In other alternate embodiments, the method can further include movingthe electromagnetic energy source to a succession of locations in a wellborehole for emitting the pulses of electromagnetic energy at thelocations for travel through a subsurface hydrocarbon reservoir. Aplurality of electromagnetic sensors can be lowered through a sensorbore in a subsurface hydrocarbon reservoir. A plurality ofelectromagnetic sensors can be located in an array over an earth surfaceabove a subsurface hydrocarbon reservoir. The pulses of electromagneticenergy can be emitted from the electromagnetic energy source to travelthrough a subsurface hydrocarbon reservoir.

In yet other alternate embodiments, the method includes forming ameasure of arrival time data of the pulses of electromagnetic energy ata plurality of electromagnetic sensors, analyzing the measure of arrivaltime data from the plurality of electromagnetic sensors to form arepresentation of subsurface features of the subsurface hydrocarbonreservoir, and forming an image of the representation of subsurfacefeatures of the subsurface hydrocarbon reservoir.

In still another alternate embodiment of this disclosure, a method forelectromagnetic imaging of a subsurface hydrocarbon reservoir includeslowering an electromagnetic energy source on a wireline in a wellborehole to a depth of interest in the subsurface hydrocarbon reservoir.The electromagnetic energy source includes a sonde assembly attached tothe wireline for travel in the well borehole, the sonde assemblyincluding a first section axially aligned with, and spaced from, asecond section. The electromagnetic energy source also includes anenergy storage capacitor formed by a conductor member of the sondeassembly, an electrode mounted in each of the first section and thesecond section of the sonde assembly and serving as an inner conductor,and a capacitive charge storage medium mounted in each of the firstsection and the second section of the sonde assembly between theconductor member and the electrode. The electromagnetic energy sourcefurther includes a fast-closing switch located between one of theconductor members and the electrodes, of the first and second sections.Pulses of electromagnetic energy are emitted with the electromagneticenergy source to travel through the sub surface hydrocarbon reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features, aspects andadvantages of the invention, as well as others that will becomeapparent, are attained and can be understood in detail, a moreparticular description of the invention briefly summarized above may behad by reference to the embodiments thereof that are illustrated in thedrawings that form a part of this specification. It is to be noted,however, that the appended drawings illustrate only preferredembodiments of the invention and are, therefore, not to be consideredlimiting of the invention's scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic section view of a transmitter-receiver array for aborehole to borehole electromagnetic survey, in accordance with anembodiment of this disclosure.

FIG. 2 is a schematic section view of an electromagnetic energy sourceand storage capacitor, in accordance with an embodiment of thisdisclosure.

FIG. 3 is a schematic cross section view of the electromagnetic energysource of FIG. 2.

FIG. 4 is a schematic cross section view of the electromagnetic energysource of FIG. 2.

FIG. 5 is a schematic section view of an electromagnetic energy source,in accordance with an embodiment of this disclosure.

FIG. 6 is a schematic cross section view of the electromagnetic energysource of FIG. 5.

FIG. 7 is a schematic section view of an electromagnetic energy source,in accordance with an embodiment of this disclosure.

FIG. 8 is a schematic cross section view of the electromagnetic energysource of FIG. 7.

FIG. 9 is a circuit diagram of the electromagnetic energy source of FIG.7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Looking at FIG. 1, an example arrangement of a transmitter-receiverarray for a borehole to borehole electromagnetic survey is shown. Thetransmitter can be electromagnetic energy source 10. Electromagneticenergy source 10 can be located within well borehole 12. Well borehole12 can extend through subsurface hydrocarbon reservoir 14.Electromagnetic energy source 10 can emit pulses of electromagneticenergy to travel through subsurface hydrocarbon reservoir 14 forelectromagnetic imaging of subsurface hydrocarbon reservoir 14.

Although one electromagnetic energy source 10 is shown in the example ofFIG. 1, in alternate embodiments, multiple electromagnetic energysources 10 can be located within borehole 12. Alternately, one or moreelectromagnetic energy sources 10 can be located at the earth surface 15above the subsurface hydrocarbon reservoir. In the example of FIG. 1, aseries of electromagnetic sensors 16 are located in sensor bore 18.Sensor bore 18 can be a borehole that extends through subsurfacehydrocarbon reservoir 14 and spaced apart from well borehole 12. Inalternate embodiments, electromagnetic sensors 16 can be in an arrayover the earth surface 15 above subsurface hydrocarbon reservoir 14.When electromagnetic energy source 10 is located in well borehole 12 andelectromagnetic sensors 16 are located over the earth surface 15, thearrangement is known as a borehole to surface array. Generally either orboth of the electromagnetic energy source 10 and electromagnetic sensors16 are located within a borehole so that the EM signals pass throughsubsurface hydrocarbon reservoir 14 when traveling from electromagneticenergy source 10 to electromagnetic sensors 16. Electromagnetic sensors16 can form a measure of the arrival time of the emitted pulses fromelectromagnetic energy source 10 to image subsurface hydrocarbonreservoir 14.

As can be seen in FIG. 1, a multitude of EM energy measurements can beperformed with different combinations of transmitter locations 20 andreceiver locations 22 in order to sample various parts of thesubterranean features from different directions, including subsurfacehydrocarbon reservoir 14. Both the electromagnetic energy source 10 andelectromagnetic sensors 16 can be a part of a downhole tool or locatedin a tool and can be movable to between a succession of locations, suchas between transmitter locations 20 and receiver locations 22.

Electromagnetic energy source 10 can be attached to source wireline 24for travel in well borehole 12 to a depth of interest. In the example ofFIG. 1, the source wireline 24 extends from vehicle 26 at the surface.System control unit 28 can be associated with vehicle 26 and can be usedto control the pulses emitted by electromagnetic energy source 10. Asecond vehicle 30 can have a receiver wireline 32 for attaching toelectromagnetic sensors 16 and for moving electromagnetic sensors 16within sensor bore 18.

Looking at FIGS. 2-3, and 5-8, electromagnetic energy source 10 includessonde assembly 34. Sonde assembly 34 has two main sections: firstsection 34 a is axially aligned with, and spaced from, second section 34b. Electromagnetic energy source 10 also includes energy storagecapacitor 40 with capacitive charge storage medium 44.

An electrode 42 is mounted in, each of first section 34 a and secondsection 34 b of sonde assembly 34. First electrode 42 a is located infirst section 34 a and second electrode 42 b is located in secondsection 34 b. Electrode 42 can be an elongated member and have a tubularshape (FIGS. 2-3), or can be a solid rod or wire (FIGS. 5-8). Electrode42 can be formed of copper, and in alternate embodiments, can be formedof silver, aluminum, gold or other suitable similar material.

The capacitive charge storage medium 44 is mounted in each of the firstsection 34 a and the second section 34 b of the sonde assembly 34.Capacitive charge storage medium 44 can be formed with materials withrelatively high dielectric permittivity, magnetic permeability, or acombination of dielectric permittivity and magnetic permeability. Thematerial can be selected so that e and mu of such material optimizestransmitter impedance to match the external medium. This increases thecapacitance and inductance of the system and decreases the groupvelocity of the pulses emitted by electromagnetic energy source 10, todefine a slow-wave antenna. Providing such a capacitive charge storagemedium 44 can materially decrease the length of an antenna structure fora given wavelength emitted. As an example, capacitive charge storagemedium 44 can be formed of a material that includes ferrite, steel,permalloy, TiO2, PZT, magnetite, other iron particles, or a mix thereof.Such particles 44 a can be mixed in an epoxy matrix 44 b. The specificcomposition of the mixture used for capacitive charge storage medium 44would depend on the properties of the reservoir materials and thegeometry of the antenna. In an example embodiment, capacitive chargestorage medium 44 can have particles 44 a with a dielectric permittivityof 100 (e), and a magnetic permeability of 100 (mu) and consist of bothTiO2 and magnetite. These particles 44 a can be located in a 1:1 mixturein an insulating epoxy matrix 44 b. This example embodiment would resultin an overall dielectric permittivity in the range of 40 (e), and amagnetic permeability in the range of 40 (mu) after considering thelinear combination of the components, per effective medium theory, andtherefore will result in an effective antenna that performs as though itis in the range of 40 times larger than the actual length of theantenna.

In the example embodiments of FIGS. 2-3 an electric field can radiateout from each electrode 42 and through the nearby capacitive chargestorage medium 44 forming energy storage capacitor 40. In the exampleembodiments of FIGS. 5-8, conductor member 33 of sonde assembly 34serves as a first conductor and capacitive charge storage medium 44 islocated between the conductor member 33 and electrode 42. Capacitivecharge storage medium 44 electrically isolates conductor member 33 fromelectrode 42.

Electromagnetic energy source 10 can further include fast-closing switch46, which is located between one of the electrode 42 (FIGS. 2, 4, and 5)or the conductor member 33 (FIGS. 7 and 9) of the first and secondsections 34 a, 34 b. Fast-closing switch 46 can be, for example, a sparkgap. When fast-closing switch is closed, such as when the spark gap isbroken down, electromagnetic energy source 10 will generate anelectromagnetic pulse. In alternate embodiments, fast-closing switch 46can include avalanche transistors, thyratrons, ignitrons,silicon-controlled rectifier, and especially triggered spark gaps.Fast-closing switch 46 can be selected to have performance metricsconcerning peak current, peak voltage, useful number of shots, jitter,complexity and geometry that will suit the environment, conditions, andperformance criteria for which the electromagnetic energy source 10 isto be used.

Electromagnetic energy source 10 can also have high voltage power supply48 connected between one of the electrodes 42 (FIGS. 2, 4, and 5) or theconductor members 33 (FIGS. 7 and 9). Electromagnetic energy source 10will be located between the same of the electrode 42 (FIGS. 2, 4, and 5)or the conductor member 33 (FIGS. 7 and 9) as fast-closing switch 46. Acomponent that is not directly connected to electromagnetic energysource 10 can act as a ground.

In the example embodiment of FIG. 2, fast-closing switch 46 is connectedbetween first and second sections 42 a, 42 b of electrode 42 and highvoltage power supply is also connected between first and second sections42 a, 42 b of electrode 42. In this example, capacitive charge storagemedium 44 acts as a ground. In such an embodiment, capacitive chargestorage medium 44 proximate to electrode 42 will form energy storagecapacitor 40 and capacitive charge storage medium 44 proximate to anouter diameter of capacitive charge storage medium 44 will act as theground.

Looking at FIGS. 5 and 6, fast-closing switch 46 is connected betweenfirst and second sections 33 a, 33 b of conductor member 33 and highvoltage power supply is also connected between first and second sections33 a, 33 b of conductor member 33. In this example, first electrode 42 aand second electrode 42 b act as grounds. Looking at FIGS. 7 and 9,fast-closing switch 46 is connected between first and second sections 33a, 33 b of conductor member 33 and high voltage power supply is alsoconnected between first and second sections 33 a, 33 b of conductormember 33. In this example, first electrode 42 a and second electrode 42b act as grounds.

Power can be provided to high voltage power supply 48 from outside ofelectromagnetic energy source 10 with pair of high resistivity leads.High impedance DC connections will reduce the amount of induced currentthat will be generated in the connections by the high current pulsethrough electrode 42 when sonde assembly 34 discharges. In alternateembodiments, the magnetic permeability property of capacitive chargestorage medium 44 to channel power as a magnetic field down the lengthof second section 34 b of sonde assembly 34 can be utilized. Secondsection 34 b of sonde assembly 34 can be associated with a pick up coillocated between first and second sections 34 a, 34 b of sonde assembly34 to harvest power at the high voltage power supply 48. In thisembodiment, the power could be generated at the top of Second section 34b of sonde assembly 34 with a coil that generates a high frequencycoaxial magnetic field. Such as configuration would provide aninductively coupled transformer, as would be evident to those skilled inthe art. Such an embodiment would eliminate any parallel conductorsalong the second section 34 b of sonde assembly 34 and there byeliminate any parasitics that would degrade the transmitted pulse.

Current limiting resistors 50 can be located between the high voltagepower supply and both of the electrode of the first section and theelectrode of the second section. Current limiting resistors 50 can blockhigh current pulses from returning up the supply wire towards highvoltage power supply 48. This will isolate the antenna system, from highvoltage power supply 48 while the electromagnetic pulse is beingemitted.

Each section 34 a, 34 b of sonde assembly 34 can have end cap 39 formedof an insulating material. The capped end of first section 34 a andsecond section 34 b can face towards each other. Electrode 42 canprotrude through end cap 39 of sonde assembly 34.

In the example embodiment of FIGS. 2-3, each section 34 a, 34 b of sondeassembly 34 can include an elongated tubular member with a central borecentered around axis Ax. Electrode 42 is centered along axis Ax of eachof first section 34 a and second section 34 b of sonde assembly 34.Electrode 42 can be located within the central bore of sonde assembly34. Electrode 42 can be an elongated member and have a tubular shape.Electrode 42 is sheathed within capacitive charge storage medium 44 sothat capacitive charge storage medium 44 surrounds electrode 42. An endof electrode 42 passes through end cap 39. Energy storage capacitor 40is formed by an electric field radiating out from electrode 42 andthrough the nearby capacitive charge storage medium 44. The amount ofenergy stored will vary with the square of the electric field. IfElectrode 42 has a small diameter, then almost all of the electric fieldpotential drop will occur inside the capacitive charge storage medium44.

Looking at the alternate example embodiment of FIGS. 5-6, conductormember 33 can be a wire that extends through each section 34 a, 34 b ofsonde assembly 34. Conductor member 33 is sheathed within capacitivecharge storage medium 44. Electrode 42 can also be a wire that extendsthrough each section 34 a, 34 b of sonde assembly 34. Electrode 42 isalso sheathed within capacitive charge storage medium 44. An end of bothelectrodes 42 and conductor member 33 pass through end cap 39. Energystorage capacitor 40 is formed by the pair of wires, which are electrode42 and conductor member 33 which can have a great potential voltagebetween them. Capacitive charge storage medium 44 between electrode 42and conductor member 33 increases the mutual capacitance of electrode 42and conductor member 33. Conductor member 33 and electrode 42 serve bothas conductor elements of energy storage capacitor 40 and as part of thetransmitting elements of electromagnetic energy source 10 for emittingthe electromagnetic pulse.

In the alternate examples of FIGS. 7-8, conductor member 33 can be anouter metallic body of sonde assembly 34. Each section 34 a, 34 b ofsonde assembly 34 can have an elongated tubular member, such as ametallic body, that is closed at one end and has an end cap 39 at anopposite end. Sonde assembly 34 can have a central bore centered aroundaxis Ax. Electrode 42 is centered along axis Ax of each of first section34 a and second section 34 b of sonde assembly 34. Electrode 42 can belocated within the central bore of sonde assembly 34 and can protrudethrough end cap 39 of sonde assembly 34. Electrode 42 can be anelongated member and be a solid rod. Conductor member 33 and electrode42 serve both as conductor elements of energy storage capacitor 40 andas part of the transmitting elements of electromagnetic energy source 10for emitting the electromagnetic pulse.

Looking at FIG. 1, in an example of operation, in order to form anelectromagnetic image of subsurface hydrocarbon reservoir 14electromagnetic energy source 10 can be mounted to, or part of, a welltool and lowered on a wireline in well borehole 12 to a depth ofinterest.

The downhole tool associated with electromagnetic energy source 10 canhave an upper section with a mechanical connector that attaches to awire line, an electrical power connection, and a synchronizing signalconnection. Such upper section and connections can be orientated likeknown current downhole wireline tools. A lower section of the downholetool can house sonde assembly 34. Electromagnetic energy source 10 canbe encased in a strong, insulating polymeric material to providestructural integrity while also allowing for the transmission ofelectromagnetic signals.

A single electromagnetic energy source 10 can be utilized, as shown inthe example of FIG. 1. Alternately, a plurality of electromagneticenergy sources 10 can be lowered in well borehole 12.

Pulses of electromagnetic energy can be emitted from the singleelectromagnetic energy source 10, or at each of the plurality ofelectromagnetic energy sources 10, as applicable, to travel throughsubsurface hydrocarbon reservoir 14 and a resulting signal can bereceived by electromagnetic sensors 16. An electromagnetic pulse withknown characteristics is generated from the high power, pulsedelectromagnetic energy source 10 from locations in or near subsurfacehydrocarbon reservoir 14. In order to generate the electromagneticpulse, high voltage power supply 48 charges up energy storage capacitor40 through current limiting resistor 50 until fast-closing switch 46 isclosed. In the Example of FIG. 2, fast-closing switch 46 is a spark gapthat is closed when the voltage exceeds the break-down voltage of thespark gap. With the fast-closing switch closed, electromagnetic energysource 10 will emit the pulse of electromagnetic energy. After theelectromagnetic pulse is emitted, high voltage power supply 48 canrecharge energy storage capacitor 40.

By combining energy storage, pulse formation and radiating elements intoa single structure, the problem of impedance matching between separatedistributed components of an electromagnetic survey system required forthese respective functions is eliminated. Systems and methods of thisdisclosure therefore eliminate the problem of load matching between apower supply, cable or transmission-line, and antenna. With the energystorage element of energy storage capacitor 40 and fast-closing switch46 both inside the transmitting antenna element of the pair of disclosedself-powered impulse antennas, the need for a cable between the powersource and the transmission element are eliminated, and reflections andlosses in the system are minimized.

A plurality of electromagnetic sensors 16 can be mounted to or part of awell tool and lowered in sensor bore 18 that extends through subsurfacehydrocarbon reservoir 14. Alternately, the plurality of electromagneticsensors 16 can be arranged in an array over an earth surface 15 abovesubsurface hydrocarbon reservoir 14. The emitted pulsed EM signal istransmitted through subsurface hydrocarbon reservoir 14 and recorded atone or more electromagnetic sensors 16 after travel through thesubsurface formations surrounding well borehole 12 and sensor bore 18.The EM signal recorded by electromagnetic sensors 16 differs from thepulsed signal emitted by electromagnetic energy source 10 incharacteristics (e.g. time, amplitude, power spectrum, etc.) that dependon the properties of the intervening medium (e.g. the reservoir) andspatial variations of those properties.

Electromagnetic energy source 10 can be moved between a succession oflocations, such as transmitter locations 20, in well borehole 12 foremitting pulses of electromagnetic energy at such locations for travelthrough subsurface hydrocarbon reservoir 14. Similarly, electromagneticsensors 16 can be moved between a succession of locations, such asreceiver locations 22, to receive the resulting signal at suchsuccession of locations. In this way, a more complete electromagneticimage can be formed of subsurface hydrocarbon reservoir 14.

Recording and processing instrumentation associated with system controlunit 28 at the surface can receive and store information relating to theresulting signal received by electromagnetic sensors 16. System controlunit 28 can also perform additional functions such as computerizedanalysis of the resulting signal, display certain results derived fromthe resulting signal, and store the resulting signal and computerizedanalysis on a computer for further processing and computerized analysis.System control unit 28 can, as an example, be used to form a measure ofthe arrival time of the emitted pulses at a plurality of electromagneticsensors, and to analyze the measure of arrival time data from theplurality of electromagnetic sensors. From this information, arepresentation of subsurface features of the subsurface hydrocarbonreservoir, and an image of the representation of subsurface features ofthe subsurface hydrocarbon reservoir, can be formed.

Embodiments of present invention thus generate information about thespatial distribution and composition of fluids in a hydrocarbonreservoir. The operation can be repeated periodically to, as an exampledetermine the direction, velocity and saturation of injected fluids,such as a water flood, or to visualize modified reservoir volume as afunction of time. This can assist in optimizing reservoir management,preventing oil bypass and thereby improving volumetric sweep efficiencyand production rates.

The invention has been sufficiently described so that a person withaverage knowledge in the matter may reproduce and obtain the resultsmentioned in the invention herein Nonetheless, any skilled person in thefield of technique, subject of the invention herein, may carry outmodifications not described in the request herein, to apply thesemodifications to a determined structure, or in the manufacturing processof the same, requires the claimed matter in the following claims; suchstructures shall be covered within the scope of the invention.

It should be noted and understood that there can be improvements andmodifications made of the present invention described in detail abovewithout departing from the spirit or scope of the invention as set forthin the accompanying claims.

What is claimed is:
 1. An electromagnetic energy source for emittingpulses of electromagnetic energy, the electromagnetic energy sourcecomprising: a sonde assembly including a first section axially alignedwith, and spaced from, a second section; an energy storage capacitorincluding: an electrode mounted in each of the first section and thesecond section of the sonde assembly and operable to generate anelectric field; and a capacitive charge storage medium mounted in eachof the first section and the second section of the sonde assembly andsurrounding each electrode; a fast-closing switch located between theelectrodes of the first and second sections of the sonde assembly; wherethe sonde assembly has a conductor member serving as a first conductorand the electrode serves as a second conductor, and the conductor memberis electrically isolated from the electrode with the capacitive chargestorage medium.
 2. The electromagnetic energy source according to claim1, wherein the capacitive charge storage medium is selected to provide adecrease of a group velocity of pulses of electromagnetic energy.
 3. Theelectromagnetic energy source according to claim 1, wherein thecapacitive charge storage medium is formed of a material that includesiron particles and an epoxy matrix.
 4. The electromagnetic energy sourceaccording to claim 1, wherein the electromagnetic energy source furtherincludes a plurality of electromagnetic energy sources emitting pulsesof electromagnetic energy to travel through a subsurface hydrocarbonreservoir.
 5. The electromagnetic energy source according to claim 1,wherein the electromagnetic energy source is movable to a succession oflocations in a well borehole for emitting the pulses of electromagneticenergy at the locations for travel through a subsurface hydrocarbonreservoir.
 6. The electromagnetic energy source according to claim 1,wherein the capacitive charge storage medium is located between theconductor member and the electrode.